This proposal seeks to test the hypothesis that there are complex factors that affect SIV and HIV-1 replication and fitness in the host that have not yet been defined using standard cell culture or immunological assays. We have identified highly related SIVMne variants that are prototype viruses from early- and late-stage infection, respectively, which show dramatic difference in their replication and pathogenic properties. Such differences make these variants valuable tools for examining why and how certain viruses are selected for survival and persistence in the face of the host's immune response. The goal of the proposed studies is to determine the mechanism(s) that confer increased fitness to the latest age variants. Interestingly, our preliminary studies suggest that the late-stage variants continue to undergo considerable selection after they are transmitted to a new host, including predicted changes in glycosylation. Glycosylation changes typically indicate humoral immune selection, but we were unable to detect neutralizing antibodies using standard assay conditions in our initial studies. Our recent preliminary studies suggest that neutralizing antibody responses may develop at later times in these animals. We propose to explore the basis for the selection of these variants of the late-stage virus, and determine what factors contribute to the continued variation and positive selection of new viruses. This will include a more detailed analysis of antibody-mediated antiviral mechanisms, including analysis of the temporal development of both neutralizing and Fc-mediated antibody responses. These data will be compared with sequence studies to determine if viruses with glycosylation changes emerge after these antibodies are detected. Our studies will also address whether viral divergence is dependent on the fitness and replication properties of the infecting virus strain. In the course of the proposed studies, we hope to define a cell culture model system that predicts the increased replication fitness observed in the animals. Such a system would be broadly useful for predicting replication capacity and fitness and associated pathogenic properties of other viral strains of SIV and HIV. Collectively these studies will define the interplay between the properties of the infecting virus, the immune response and replication fitness in SIV/HIV pathogenesis.